/Engine/Source/Runtime/Slate/Public/Framework/Application/MenuStack.h
Notifies the stack that a new window was activated. The menu stack will be dismissed if the activated window is not a menu in the stack
void OnWindowActivated ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > ActivatedWindow )